An MMC is a converter that uses a switching element, such as an insulated-gate bipolar transistor (IGBT), etc., which is on/off controllable, and is capable of outputting a voltage equal to or greater than a withstand voltage of the switching element. The MMC is applied in high voltage direct current (HVDC), a STATCOM, a motor drive inverter, etc.
The MMC for HVDC is composed of multiple converter arms, and each converter arm is composed of multiple submodules connected to each other in series. Each submodule includes, for example, an energy storage unit and a power semiconductor. The power semiconductor may include a power semiconductor switch and a freewheeling diode, and may be realized as, for example, an IGBT. In such a submodule, multiple power semiconductors are connected to form a so-called half-bridge or full-bridge circuit.
An HVDC system connected to such an MMC, in order to increase a rate of operation, generally includes a redundancy submodule as a spare in addition to a submodule required at a rating in preparation for a breakdown of the submodule. Thus, when the breakdown of the submodule in operation occurs, the redundancy submodule is applied such that the system may run without interruption. Accordingly, the redundancy submodule operates instead of the submodule which has broken down such that stable operation of the system may be realized.
However, in the case where there is no more redundancy submodule to be applied due to continuing breakdown of the submodule in operation, when the additional breakdown of the submodule in operation occurs, voltage imbalance between multiple phases causes interruption of the system and affects an electric power system for HVDC.